Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about 500 to about 1,000 times its own weight, and each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of hygiene products such as paper diapers for children or sanitary napkins, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.
In most cases, these super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. For these applications, the super absorbent polymer should exhibit a high moisture absorbency, it should not release the absorbed water even in the external pressure, and additionally it should well retain the shape even in a state where the volume is expanded (swelled) by absorbing water, and thereby exhibit excellent liquid permeability.
Particularly, in recent years, with the development of hygienic materials such as diapers in which the content of pulps has been reduced or pulps have been eliminated, there is an increasing technical demand for further improving the absorbency under pressure and the liquid permeability of the super absorbent polymer. In this regard, there have been many attempts from the past to improve the surface gel strength/hardness of the super absorbent polymer, to inhibit the blocking between the super absorbent polymer particles and to further improve the absorbency under pressure and the liquid permeability of the super absorbent polymer.
Typically, attempts have been made to use silica particles such as fumed silica during surface crosslinking or after surface crosslinking, in order improve the gel strength of the super absorbent polymer, to inhibit the blocking and to improve the absorbency under pressure and the liquid permeability of the super absorbent polymer.
However, since the silica particles were difficult to uniformly distribute on the surface cross-linked layer because the aggregation between particles is large and the dispersibility of a surface crosslinking liquid is very low. As a result, the improvement of the absorbency under pressure and the liquid permeability of the super absorbent polymer due to the use thereof has been limited.
Due to the above-mentioned problems, attempts to further improve the absorbency under pressure and the liquid permeability of the super absorbent polymer are continuously performed.